1. Field of the Invention
The present invention relates to next generation wireless communication systems; and more particularly, to methods for flow control in channels of these systems.
2. Description of Related Art
In telecommunications systems, particularly in the well-known High Speed Downlink Packet Access (HSDPA) specification in the Universal Mobile Telecommunication System (UMTS) standard, for example, a transport channel such as a High Speed Downlink Shared Channel (HS-DSCH) is used for transmissions from a source device (e.g., Node-B or base station) to one or more destination devices (e.g., mobile stations or user equipment (UE)).
Typically, flow control in wireless communications systems is necessary in order to regulate a transmission over the transport channel from the source device to the destination device (e.g., from UE to Node-B). Flow control techniques should account for the receiving capability of the destination device, so that queuing at the destination device is managed so as to avoid congestion and packet loss at the destination device. With flow control, both the transport latency and signaling latency is reduced, since retransmissions due to discarded packets as a result of congestion or buffer overflow at the destination device is reduced or avoided.
In HSDPA, downlink flow of data to the UE needs to be regulated for similar reasons, i.e. to avoid the congestion and overflowing of the buffer at the UE. The problem is more serious in HSDPA due to a fast channel dependent scheduling nature that is present at the Node-B.
In HSDPA, in order to improve efficiency, the scheduling function has been relocated from a Radio Network Controller (RNC, a central, fixed controller at the UMTS core for example that is responsible for managing system radio resources) to the base station or Node B in order to provide “fast” scheduling based on channel quality feedback from the UEs. Moreover, technologies such as adaptive modulation and coding (AMC) and hybrid automated repeat request (HARQ) have been introduced to improve overall system efficiency and capacity by providing higher data rates and greater transmission robustness. In general, a scheduler selects a UE for transmission at a given time, and adaptive modulation and coding allows selection of the appropriate transport format (modulation and coding) for the current channel conditions seen by the UE.
In HSDPA, for example, the scheduler, AMC and HARQ functions are provided by a MAC-hs (medium access control-high speed) controller located in a base station. The MAC-hs is responsible for handling the data transmitted on the air interface. Furthermore the MAC-hs has responsibility to manage the radio link physical resources allocated to HSDPA. In general, the functions carried out by MAC-hs include scheduling/priority handling, Hybrid ARQ, and a physical layer transport format, e.g., modulation, coding scheme, etc. The flow control entity would also reside in the MAC, for HSDPA in the MAC-hs.
Thus, in a fast channel dependent scheduling scheme, large amounts of data may be sent to the UE when the UE channel condition is very good. Accordingly, flow control mechanisms or techniques should be able to inform a Node-B as to the current buffer status, or the receiving capability at the UE, in order to avoid overflow or congestion conditions at the receiver buffer in the UE.
The following illustrates an example where an overflow condition could occur. A UE may be used as a personal gateway, e.g., where the UE is used to connect to other devices such as laptop, personal display assistant (PDA), etc., through a BLUETOOTH wireless protocol, for example, which allows users to make effortless, wireless and instant connections between various external communication devices, such as mobile phones and desktop and notebook computers. The radio link between the UE and the connected external device could be temporarily reduced, since bit rate is lowered as interference increases. As a result, the data enroute to the external device is held up at the UE. To control the buffer overflow at the UE, the present link setup between a Radio Resource Controller (RRC) at the Node-B and the UE has to be reconfigured. Before such a reconfiguration can take place, the Node-B will already have scheduled additional data to the UE. The UE will have no choice but drop the packets meant for the destination device once the buffer at the UE is full. Since packets have already been correctly transmitted by the HARQ, any retransmission incurs additional delay, thereby wasting radio resources as well.
Flow control techniques must be able to avoid these overflow conditions. Equally important, flow control must be able to perform selective flow control on the multiple flows originating from the source device. More than one “flow” or application can be multiplexed in the downlink transmission to the UE. These flows could be of different priority levels or classes from different or same applications. Up to a maximum of eight priorities classes are supported in HSDPA. Thus, if there is more than one data flow existing for the UE, and buffer occupancy is critical for only one of the flows, flow control that is able to differentiate between these different flows is important in order to maximize system throughput.
Presently, flow control mechanisms employing an out-of-band signaling solution have been proposed for UE flow control for both uplink signaling and downlink signaling. In general, out-of-band signaling sends the control messages in the control channel or channels. Out-of-band signaling carries physical layer, or layer 1-generated messages. These messages, without additional signaling defined therein, are essentially blind to the contents of the data payload.
Flow control mechanisms using in-band signaling could be embodied in the following possible combinations: (1) Out-of-band Uplink and Downlink Signaling; (2) Out-of-band Uplink and In-band Downlink Signaling; and (3) In-band Uplink, and In-band/Out-of-band Downlink Signaling.
In general, uplink signaling consists of transmitting UE feedback to the Node-B request for a reduction of flow rate, for termination of flow, or for the postponement or hold from the Node-B, or the restart of flow from the Node-B to the UE. A UE would send such a request before its application buffer is full to avoid dropping any packet caused by buffer overflow. The Node-B will reduce, terminate or hold the data flow to the UE for a predefined time, or until the Node-B receives a restart flow request from the UE.
In addition to the uplink UE flow control request signaling from the UE, out-of-band downlink signaling from the Node-B carrying the acknowledgment message of the flow control request by the UE can be employed in the downlink to increase the reliability of the flow control mechanism. Errors in the uplink UE requests for flow control may lead to unrecoverable error cases. In HSDPA, where enhancement to the air interface capability is only in the downlink, the uplink is relatively more error prone since no HARQ process and AMC are implemented. The UE can use a higher power on the uplink request. However, it may not be possible to increase the power due to system UE power limitations. Moreover, higher transmit power from the UE will generate more interference, thus affecting the overall uplink capacity for the system.
In the uplink UE request for flow control, either in-band or out-of-band signaling could conceivably be employed. However, presently in HSDPA, where no new uplink transport channel has been proposed, the choice of signaling method is restricted to only the above-described out-of-band signaling method.
Further, in the downlink, there are several disadvantages with using the aforementioned out-of-band flow control methods for UE flow control: (a) flow control for the different flows cannot be provided since knowledge of the data payload types is not available at the layer 1 or the physical layer. Hence, flow control has to be performed for all the flows sent to the UE; (b) out-of-band signaling requires protection through more complex coding and/or modulation schemes that requires higher power requirements. These complex schemes consume precious air interface resources that otherwise could be used for other functions; and (c) in HSDPA, the out-of-band signaling does not gain from the HARQ retransmissions process that, among many of its advantages, also provides robustness at high Doppler and provides a Turbo Coding gain that is not employed by out-of-band signaling techniques. Accordingly, flow control mechanisms or techniques that do not distinguish between the different priority flows must simultaneously control each of the different priority flows to the UE.